Have you ever gotten a plea to wire money to a friend stranded at an international airport? An oddly written message from someone you haven’t heard from in ages? Compared to five years ago, more scams, illegal, fraudulent or spammy messages today come from someone you know. Although spam filters have become very powerful—in Gmail, less than 1 percent of spam emails make it into an inbox—these unwanted messages are much more likely to make it through if they come from someone you’ve been in contact with before. As a result, in 2010 spammers started changing their tactics—and we saw a large increase in fraudulent mail sent from Google Accounts. In turn, our security team has developed new ways to keep you safe, and dramatically reduced the amount of these messages.

Spammers’ new trick—hijacking accounts
To improve their chances of beating a spam filter by sending you spam from your contact’s account, the spammer first has to break into that account. This means many spammers are turning into account thieves. Every day, cyber criminals break into websites to steal databases of usernames and passwords—the online “keys” to accounts. They put the databases up for sale on the black market, or use them for their own nefarious purposes. Because many people re-use the same password across different accounts, stolen passwords from one site are often valid on others.

With stolen passwords in hand, attackers attempt to break into accounts across the web and across many different services. We’ve seen a single attacker using stolen passwords to attempt to break into a million different Google accounts every single day, for weeks at a time. A different gang attempted sign-ins at a rate of more than 100 accounts per second. Other services are often more vulnerable to this type of attack, but when someone tries to log into your Google Account, our security system does more than just check that a password is correct.

Legitimate accounts blocked for sending spam: Our security systems have dramatically reduced the number of Google Accounts used to send spam over the past few years

How Google Security helps protect your account
Every time you sign in to Google, whether via your web browser once a month or an email program that checks for new mail every five minutes, our system performs a complex risk analysis to determine how likely it is that the sign-in really comes from you. In fact, there are more than 120 variables that can factor into how a decision is made.

If a sign-in is deemed suspicious or risky for some reason—maybe it’s coming from a country oceans away from your last sign-in—we ask some simple questions about your account. For example, we may ask for the phone number associated with your account, or for the answer to your security question. These questions are normally hard for a hijacker to solve, but are easy for the real owner. Using security measures like these, we've dramatically reduced the number of compromised accounts by 99.7 percent since the peak of these hijacking attempts in 2011.

Help protect your account
While we do our best to keep spammers at bay, you can help protect your account by making sure you’re using a strong, unique password for your Google Account, upgrading your account to use 2-step verification, and updating the recovery options on your account such as your secondary email address and your phone number. Following these three steps can help prevent your account from being hijacked—this means less spam for your friends and contacts, and improved security and privacy for you.